Sentinel Peak, Mountain summit in Tucson, United States.

Sentinel Peak is a volcanic mountain summit in the Tucson Mountains west of the Santa Cruz River, rising approximately 884 meters above sea level. The peak consists of multiple layers of volcanic rock that once extended throughout the current Tucson Basin area.

Spanish forces established a sentinel post on the peak in 1775 near the Presidio San Agustín del Tucson to watch for Apache warriors. The strategic location served as a defense point for the early Spanish settlement in the region.

The block "A" was completed on March 4, 1916, not 1914: students hauled basalt from a quarry at the mountain's base and whitewashed the 70 ft wide, 160 ft tall letter on the east face. It has traditionally been painted white, but was black in 2003, then red, white and blue by city council decision, later restored to white, and green for St. Patrick's Day. During Territorial Cup week, rival fans have repainted the opposing school's "A".

Sentinel Peak lies southwest of downtown Tucson, west of the Santa Cruz River, and is reached by road up to the summit area, where parking puts visitors near the "A" and overlooks. The 272-acre park is the largest natural-resource park in the city. The east face bearing the "A" is best seen from the city side; summer heat makes early visits preferable.

The "A" was whitewashed on March 4, 1916, after a civil engineering student on the 1914 football team turned the survey and design into a class project. Its basalt came from a quarry at the mountain's base that also supplied stone for the University of Arizona campus walls. The letter has since been repainted black, green, red and blue for protests and causes.